Dynamic Characteristics Of Dry Gas Seal Influenced By The Real Gas,Choked Flow And Inertia Effects

With the development of industrialization,the climatic pollutions have been the focus of global attention.Thus,how to reduce the emissions of environmental impact gases（e.g carbon dioxide,volatile organic compounds,etc.）has been attracting more and more attention.Industrial production,such as petroleum processing,energy and power industry,etc.,are the main emission source of environmental impact gases.Strengthening the application of sealing technology in industrial processes and then controlling environmental impact gases at the source could achieve a purpose of emission reduction,which is an effective measure to improve the current atmospheric environment.Dry gas seal（DGS）is a fast-growing sealing technology which plays an important role in promoting environment pollution control by improving its operation stability and reducing leakage.As the core component of rotary machineries,the main reason of sealing failure is dynamic instability and rotary machineries are gradually applied in high-pressure conditions,which proposes higher demand for sealing operation reliability and low leakage.To provide more practical theoretical guidance for the achievement of these objectives,it is necessary to stretch and enrich the existed dynamic theory of DGS.Aiming at the fact that lubricating gas properties deviated from ideal gas greatly under high pressure,the mechanism that the dynamic characteristics of DGS influenced by real gas effect has been researched.In the meantime the choked flow and inertia effects neglected in conventional theoretical design are both taken into account,which to be established a set of universal dynamic research methods for high pressure DGS.Taking four common used lubricating gases（nitrogen,carbon dioxide,hydrogen and air）as the example,the applicable real gas state equations of four gases are preliminarily summarized by consulting the literature and then the density expression accuracy of these state equations are calculated.Within the range of researched condition,the optimal theoretical expression of density for each lubricating gas has been screened by comparing with NIST database.Introducing the real gas state equation into transient Reynolds equation,a expression of dynamic compressibility factor is derived and the gas film dynamic coefficients with three degrees of freedom perturbation are obtained based on small perturbation method.On the basis of these coefficients,the motion equations of DGS system,including flexible support,stationary ring and gas film,have been established,which are solved by the analytical method and complex model method respectively and the developed model is validated.Comparing with the results of ideal gas model,the mechanism that real gas effect impacts on DGS dynamic characteristic is analyzed and this influence varies with operating parameter is discussed.The results showed that the influence of real gas effect on DGS dynamic characteristic enhances with operating parameter increasing,and the influence of real gas effect on the tracking ability of stationary ring mainly depends on the deviation between compression factor and 1,i.e.real gas effect has a positive influence on the tracking ability of DGS in which the compressibility factor of lubricating gas is smaller than 1（e.g.CO₂）,while it is going in the opposite direction for those DGSs whose lubricating gas compressilility factor is larger than 1（e.g.N₂,H₂ and air）.The static pressurization experiment indicated that the variation of equilibrium film thickness with entrance pressure is in line with the theoretical calculation result and the equilibrium film thickness clacuated by the real gas model is closer to that by experiment.Then keeping entrance pressure unchanged and the root mean square error of film thickness amplitude is used to express the tracking ability of stationary ring,gas film thickness changing with rotational speed and a comparison between the experiment and the theoretical data（considering the adaptive adjustment of gas film）are analyzed.Results show that the experimental data verify some relevant conclusions of theoretical research.Since the perturbed motions in three degrees of freedom can be simplified as the two independent perturbations,an axial movement and an angular wobble around the two orthogonal axes,the critical conditions of DGS self-excitation at each freedom are obtained by eigenvalue analysis.Furthermore,the influence of real gas effect on the self-excitation stability threshold of DGS is explored.As to the case in this thesis,based on single factor optimization theory,the optimal range of spiral groove structural parameters are given by taking critical frequency ratio of angular self-excitation as the objective.With consideration of choked flow condition（i.e.gas exit speed reaching to the sound speed）,the influence of real gas effect on choked exit pressure and critical entrance pressure are studied.Then,the influence of choked flow effect on opening force and leakage changing with entrance pressure and squeeze number are also analyzed.Taking the exit velocity calculated by the dynamic pressure and the gas film as the selection criterion of steady-state pressure outlet boundary,a dynamic arithmetic of DGS considering choked flow is proposed for axial perturbation.The result showed that the choked flow effect causes inhibition to the axial tracking ability of stationary ring.A DGS pressure governing equation considering centrifugal inertia force is established and then the influence of inertia effect on steady gas film pressure,opening force and leakage for pumping-in and pumping-out carbon dioxide DGSs are analyzed.In the meantime,the dynamic gas film pressure and the tracking ability of stationary ring influenced by the inertia effect are studied under real gas model.In addition,the analysis that taking dynamic leakage as the evaluation parameter showed that there exist complex interactive actions among the three special effects.